A technique by which a transistor is formed using a semiconductor thin film formed over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor whose active layer includes an amorphous oxide including indium (In), gallium (Ga), and zinc (Zn) is disclosed (see Patent Document 1).
Transistors including oxide semiconductors have on-state characteristics (e.g., on-state current) superior to those of transistors including amorphous silicon. In order to apply the transistors including oxide semiconductors to high-performance devices, such transistors are required to have further improved characteristics, and thus techniques of crystallization of oxide semiconductors have been developed (Patent Document 2). In Patent Document 2, a technique in which an oxide semiconductor is crystallized by heat treatment is disclosed.